Texture mapping with multiple UV maps
First unwrap the cube (I used Follow Active Quads).
Add the default Color grid image texture as the image.
Add a new UV map (I called it decal). In order to do that open Properties Editor > Data tab > UV maps rollout and press "+" button. Select the face you want the decal on and press U >Unwrap.
Add another image texture in the second texture slot, set the UV map to decal and the set the Extension to Clip.



Both image textures are on the same material. The material is not specific to any face, it is on the entire model. The UV maps control where the textures go.

Here is the result, default Color grid as the texture and the blender logo as the decal.